Zeosil-Nanoblöcke: Baueinheiten in dernPr4N+-gesteuerten Synthese von Zeolith MFI

The catalytic activity of zeolites [1] often is limited by the diffusion of reagents and reaction products through the framework pore network. Recently, several efforts have been made to reduce the diffusion path length in zeolites by confining crystal growth to a nanometer length scale [2][3][4][5][6][7][8] or by providing intracrystal mesopores during synthesis.[9-13] The latter strategy is intrinsically appealing, in part, because it precludes the need for colloidal crystal formation and avoids the drawbacks associated with nanoparticle processing. Moreover, the presence of mesopores in zeolite crystals offers the possibility of improving product selectivity in the catalytic cracking of polymeric molecules. [14] The embedding of carbon nanoparticles and certain polymers into zeolites crystals during synthesis has been shown to be an effective means of generating intracrystal mesopores.[9-13] These templating methods provide much better control of pore size than conventional steaming and chemical leaching approaches to mesopore formation in zeolites.[15] However, particle templates typically afford average pore sizes that are too large (! 10 nm) and pore size distributions that are too broad (> 10 nm widths at half maximum) for catalytic cracking reactions with high product selectivity.Herein we report a method to prepare templated zeolites with small intracrystal mesopores (average pore size 2.0-3.0 nm) and narrow pore size distributions (ca. 1.0-1.5 nm width at half maximum). We selected the MFI zeolite ZSM-5 to illustrate our approach, in part, because it is widely recognized for its unique properties as a catalyst for NO x reduction, Fisher-Tropsch chemistry, and toluene disproportionation, and as an additive for petroleum cracking. Scheme 1 illustrates our synthetic strategy for templating uniform mesopores within a zeolite matrix. In this scheme, a silane-functionalized polymer is used as a porogen for the formation of intracrystal mesopores. The presence of -SiO 3 units on the polymer allows it to be integrated into a silicaalumina sol-gel reaction mixture. Nucleation of the zeolite phase in the presence of the silylated polymer is accompanied by the grafting of the polymer to the zeolites surface through covalent Si-O-Si linkages. As the zeolite crystal grows, the

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MFI Zeolite with Small and Uniform Intracrystal Mesopores

Wang

Pinnavaia

2006

Angewandte Chemie

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“…Each of these techniques reveals another "part of the puzzle" of the formation of a crystal from a precursor solution. Aggregation of these particles to form crystals is the main aspect in some models, [12][13][14] but it has also been suggested that crystal nucleation and growth proceeds through the addition of monomers to smaller clusters. [6][7][8] Herein, we extend this work to solution conditions, from which zeolite nucleation and crystallization occurs during the MS experiments.…”

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“…[6][7][8] Herein, we extend this work to solution conditions, from which zeolite nucleation and crystallization occurs during the MS experiments. [14] The other models would involve a rather broad variety of silicate species. The formation of specific structures is directed by organic additives, tetrapropylammonium (TPA) in the case of the MFI structure and tetrabutylammonium (TBA) for MEL.…”

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“…The formation of specific structures is directed by organic additives, tetrapropylammonium (TPA) in the case of the MFI structure and tetrabutylammonium (TBA) for MEL. [14,17] We investigated nucleating silicate solutions leading to silicalite-1 or silicalite-2 under the structure-directing influence of tetra-n-propylammonium hydroxide (TPAOH) and tetra-n-butylammonium hydroxide (TBAOH), respectively, with ESI-MS. We applied dynamic light scattering (DLS) as an additional technique to monitor particle evolution in the range where ESI-MS fails (particle diameter > 1 nm). There is strong evidence that crystalline structures are formed directly from colloidal particles that serve as a nutrient [9][10][11] -at least for some cases including silicalite-1 and silicalite-2.…”

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Monitoring the Nucleation of Zeolites by Mass Spectrometry

Pelster¹,

Kalamajka²,

Schräder³

2007

Angewandte Chemie

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The early stages of crystal formation are among the most elusive problems in zeolite science and possibly in all of solidstate chemistry. [1][2][3][4] This is largely because the nucleation step or nucleation phase causes major problems in experimental investigations, as the size of the aggregates most probably involved lies between small molecules and the extended solid.[5] Often-used techniques are NMR spectroscopy, diffraction, scattering with different types of radiation, transmission electron microscopy (TEM), and X-ray absorption spectroscopy. Each of these techniques reveals another "part of the puzzle" of the formation of a crystal from a precursor solution. In previous works we introduced electrospray ionization mass spectrometry (ESI-MS) as a suitable technique to investigate the oligomer distribution and its temporal development in prenucleating silicate solutions.[6-8] Herein, we extend this work to solution conditions, from which zeolite nucleation and crystallization occurs during the MS experiments. The results suggest that it is not single silicate oligomers that play a special role in zeolite formation, but that all silicate species present in solution act as a reservoir for the formation of a solid, finally crystalline phase.We concentrated on the all-silica versions of ZSM-5 (MFI topology) and ZSM-11 (MEL topology), silicalite-1 and silicalite-2, respectively, which can both be crystallized from clear solutions. The formation of specific structures is directed by organic additives, tetrapropylammonium (TPA) in the case of the MFI structure and tetrabutylammonium (TBA) for MEL. It is unclear at which stage in the solid-state formation process the template ions exert their structure-directing effect and how this effect is actually brought about on a molecular level. There is strong evidence that crystalline structures are formed directly from colloidal particles that serve as a nutrient [9][10][11] -at least for some cases including silicalite-1 and silicalite-2. Other models involve the existence of small particles (below 10 nm) as crucial species in the formation of high-silica zeolites, although the further course of the reaction is again disputed. Aggregation of these particles to form crystals is the main aspect in some models, [12][13][14] but it has also been suggested that crystal nucleation and growth proceeds through the addition of monomers to smaller clusters. [15,16] Among the models that involve a particle-aggregation mechanism, there is disagreement whether the small entities that lead to zeolite formation have the same molecular structure and are essentially already small fragments of the zeolite framework, or whether they are only uniform in size and shape but disordered on the molecular level. The most specific model suggests one majority silicate species to be present in solution, which already exhibits the crystal structure of the final zeolite.[14] The other models would involve a rather broad variety of silicate species. [12,13] The results obtained with 29 Si NMR spectrosc...

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Novel nanostructured functional materials can be constructed by encapsulation of optically active guests in microporous hosts. [6][7][8][9][10] Nanoscale amorphous gel particles (with a size of 5-50 nm) are formed in the precursor solutions before longrange crystalline order is established. To achieve this goal, it is essential to accommodate specific molecules within the voids of the zeolite structures and to create nanosized assemblies such as thin films, layers, or monolith structures for specific applications.

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In Situ Incorporation of 2‐(2‐Hydroxyphenyl)benzothiazole within FAU Colloidal Crystals

Mintova¹,

Waele²,

Schmidhammer³

et al. 2003

Angewandte Chemie

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Novel nanostructured functional materials can be constructed by encapsulation of optically active guests in microporous hosts. Incorporation of organic molecules inside the cages of zeolites permits the control of both their molecular and optical properties. To achieve this goal, it is essential to accommodate specific molecules within the voids of the zeolite structures and to create nanosized assemblies such as thin films, layers, or monolith structures for specific applications. Generally, the microporous materials used for the preparation of nanostructured assemblies are synthesized in colloidal form from clear aluminosilicate or aluminophosphate solutions, which results in a mean particle size in the range of about 20-600 nm. [1][2][3][4][5] Nanoscale organosilicate clusters in the precursor solutions used for the synthesis of colloidal zeolites have been observed with different techniques. [6][7][8][9][10] Nanoscale amorphous gel particles (with a size of 5-50 nm) are formed in the precursor solutions before longrange crystalline order is established. [3,4] Based on these insights, the encapsulation of additional organic molecules in the crystalline internal voids of zeolite structures should be possible by direct incorporation from the precursor colloidal solutions. Moreover, colloidal molecular sieves with particle sizes in the nanometer range can have high colloidal stability in different solvents with respect to further agglomeration and sedimentation. These features make the encapsulation of functional optical molecules in nanoscale zeolite suspensions an attractive synthetic goal. Such suspensions are 1) promising systems for optical investigations of host-guest interactions, and 2) interesting precursors for the construction of nanostructured assemblies such as optical coatings and selective chemical sensors. The application of nanosized zeolites opens up possibilities for the preparation of homogeneous zeolite films on many different supports, as well as inert substrates, by efficient spin-coating of stable colloidal solutions. The generation of defined porous nanocrystals with encapsulated, optically active guests provides a route to twodimensional functional constructs for further applications, for example, in the area of optical sensors.Molecules with excited-state intramolecular proton-transfer (ESIPT) properties constitute a promising family of potential guests in functionalized zeolite materials for applications in the area of UV filtering, [11] sensing, [12] or even molecular switching. Among the different ESIPT systems, the HBX dyes (X = S,O or NH for 2-(2-hydroxyphenyl)benzothiazole (HBT), -benzooxazole, or -benzimidazole systems, respectively) have received particular attention, and their properties are being extensively studied regarding their steady-state [13][14][15][16][17][18] and time-dependent behavior.[19] The photophysical properties of these compounds are strongly dependent on the polar and protic character of the solvent that influences the relative stability of different tautom...

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Zeosil-Nanoblöcke: Baueinheiten in dernPr4N+-gesteuerten Synthese von Zeolith MFI

Cited by 39 publications

References 18 publications

MFI Zeolite with Small and Uniform Intracrystal Mesopores

MFI Zeolite with Small and Uniform Intracrystal Mesopores

Monitoring the Nucleation of Zeolites by Mass Spectrometry

In Situ Incorporation of 2‐(2‐Hydroxyphenyl)benzothiazole within FAU Colloidal Crystals

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